Furoic acid preparation method

ABSTRACT

A method is for the preparation of furoic acid or of one of its derivatives of formula (I): 
     
       
         
         
             
             
         
       
     
     in which R 1 , R 2 , R 3  and R 4  represent, independently of each other, a hydrogen atom, a linear or branched C 1 -C 6  alkyl group, a —C(═O)—H group or a —COOH group, by heterogeneous catalytic oxidation of furfural or a derivative thereof of formula (II). The oxidation is carried out in the presence of a supported catalyst based on gold nanoparticles, and in a non-alkaline aqueous medium. A composition useful in the method includes at least furfural and supported gold nanoparticles.

PRIORITY AND CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. National Phase Application under 35 U.S.C.§ 371 of International Application No. PCT/EP2017/056264, filed Mar. 16,2017, designating the U.S. and published as WO 2017/158106 A1 on Sep.21, 2017, which claims the benefit of French Application No. FR 1652217,filed Mar. 16, 2016. Any and all applications for which a foreign or adomestic priority is claimed is/are identified in the Application DataSheet filed herewith and is/are hereby incorporated by reference intheir entireties under 37 C.F.R. § 1.57.

FIELD

The present invention relates to furoic acid preparation methods.

SUMMARY

The present invention relates to a heterogeneous catalytic method forthe preparation of furoic acid or a derivative thereof from furfural orone of its derivatives in the liquid phase.

DETAILED DESCRIPTION

The present invention relates to a heterogeneous catalytic method forthe preparation of furoic acid or a derivative thereof from furfural orone of its derivatives in the liquid phase.

Furoic acid (also known as 2-furoic acid, or furan-2-carboxylic acid, oralpha-furoic acid) is an important compound for industry and may be usedin various fields of application.

In particular, furoic acid may be used as a preservative inpasteurization and sterilization steps in the field of food, thus actingas a bactericidal and fungicidal agent. It may also be used as aflavoring agent.

This compound may be useful in the preparation of nylons in the field ofbiomedical research.

Furoic acid also provides furoic acid esters and is frequently used asan intermediate in the chemical, pharmaceutical and agrochemicalindustries.

Finally, it may eventually play an important role in the field ofoptical technologies because the polar organic crystals that it forms inthe solid state could be key elements of future photonic technologiesallowing the storage of images and information (Uma et al.,Optik—International Journal for Light and Electron Optics, 2013,124(17), 2754-2757).

Among furoic acid derivatives, mention may be made, in particular, of2,5-furanedicarboxylic acid (also known by the abbreviation FDCA) whichmay be particularly useful in polymerization reactions in order toobtain, for example, polyesters, polyamides and polyurethanes. It mayalso be used in pharmacology.

Furoic acid may also be easily hydrogenated to form tetrahydrofuricacid, which is a very important intermediate in the pharmaceuticalindustry.

There is therefore a real interest in an economically-advantageousmethod for the preparation of furoic acid derivatives.

One conventional approach is to oxidize the furfural in an alkalinemedium in the presence of a catalyst. It should be noted that the use ofan alkaline medium is necessary to achieve good catalytic performance inthe oxidation reaction.

As an illustration of this approach may be mentioned, in particular, thealkali oxidation of furfural with the aid of metal catalysts in thepresence of gaseous oxygen (O₂) (Tian et al., Molecules, 2008, 13(4),948-957; Harrisson et al., Org Synth., 1956, 36, 36, and the documentJP26001111B4 of Terai et al., or with the help of chromate salts (Hurdet al., 1. Am. Chem. Soc., 1933.55(3), 1082-1084, and Chakraborty etal., Synthetic Communications, 1980, 10(12), 951-956), or in thepresence of hydrogen peroxide (H₂O₂) (Corma et al. Chem. Rev., 2007,107(6), 2411-2502) or alternatively carbon-supported Pt—Pb bimetalliccatalysts (Corma et al., Chem Rev., 2007, 107(6), 2411-2502); Verdegueret al., J. Chem. Biotechnol., 1994, 61, 97-102, and Verdeguer et al., J.Chem. Biotechnol., 1994, 112, 1-11).

Although these methods lead, for the most part, to high yields of furoicacid salts, they are unfortunately not totally satisfactory.

Thus, they have the major drawback of requiring at least one separationand conversion step subsequent to the oxidation step in order to obtainfuroic acid free of its alkaline salt. This approach therefore leads tothe production of large quantities of salts that are generally of littleor no value.

It has also been shown that the metal catalysts used are easily poisonedduring this step, making them inactive. In addition, the leaching ofmetal particles responsible for a gradual deactivation of the catalystsis also very often noted.

It has therefore been noted that the need to carry out the reaction inan alkaline medium has the undesirable side effect of deactivating thecatalyst, in particular because of the degradation of the catalystsupport.

Thus, in industrial practice, furoic acid is synthesized using AgO/Cu₂0catalysts. However, such a method suffers from the need for a highcatalyst load, the use of a diluted medium, and the existence of sidereactions. Furfural undergoes not only oxidation to furoic acid, butalso secondary reactions resulting from cleavage of the furoic cycle. Inaddition, the catalyst must be periodically regenerated insofar as theCu₂O phase is not stable.

Another approach to the chemical synthesis of furoic acid imposes apreliminary Canizzaro reaction from furfural in an aqueous NaOH solutionto obtain furfuryl alcohol and sodium 2-furanecarboxylate. The next stepis the reaction of sodium 2-furanecarboxylate with sulfuric acid toobtain furoic acid. The major disadvantage of this method is thelimitation of the theoretical yield of furoic acid to 50% and thelarge-scale generation of sodium hydrogen sulphate which must be removedfrom the reaction mixture.

Furoic acid may also be synthesized via biotechnological methods (Perezet al., African Journal of Biotechnology, 2009, 8(10), 2279-2282, Eilerset al., Planta, 1970, 94, 253-264; Luna et al., Rev. Mex. CienciasFarmacéuticas, 1997, 28, 17-19, and the documents CU22371A1 andWO9308293A1) using the action of certain microorganisms or fungi amongwhich one may cite mushrooms such as those of the species Neurosporacrassa and Neurospora ascospora, yeasts such as Saccharomycescerevisiae, and bacteria such as those of the genus Acetobacter,Bacillus, Zooglea, Nocardia and Pseudomonas.

Thus, by way of example, furoic acid may be prepared by oxidation offurfural using a biocatalytic microbial preparation with NocardiacoraHina B-276 (Perez et al., African Journal of Biotechnology, 2009,8(10), 2279-2282). Experiments involving this microbial conversionresulted in high yields, i.e. 88% from furfural. The oxidation withNocardia corallina was considered to be interesting insofar as the useof most other microorganisms leads to the production of two oxidationproducts, namely the corresponding acid and alcohol. In addition, nodestruction of the furan cycle was observed. However, as explainedbelow, drawbacks remain.

The methods currently considered for preparing furoic acid from furfuraltherefore do not give complete satisfaction. In the case ofbiotechnological methods, the main drawbacks are the complexity ofimplementation of the method, the separation of the final products fromthe mixture of reagents and also the need to use substrates of highpurity and a very diluted medium. In addition, in the case of theaforementioned Nocardia coraHina B-276 cells, the yield of furoic acidis only 88% and is obtained after 8 hours of reaction.

The most important drawbacks of the existing chemical methods arerelated, in particular, to the need to work in an alkaline medium. Asmentioned above, this constraint has the undesirable effect, on the onehand, of requiring a subsequent treatment of secondary and/orintermediate products while, on the other hand, of affecting thestability of some of the catalysts.

There is, therefore, a particular interest for a heterogeneous catalysismethod without the aforementioned drawbacks.

Thus, one of the objectives of the invention is to propose a method inheterogeneous catalysis, making it possible to directly produce furoicacid or one of its free derivatives in water (and not in the form ofalkaline salt) with a very high yield.

Another objective of the invention is to provide a heterogeneouscatalysis method that does not require working in an alkaline medium,and thus makes it possible to overcome the phenomenon of degradation ofthe catalyst support and the loss of metal residues generallyencountered under alkaline conditions.

Another object of the present invention is to provide a heterogeneouscatalysis method for obtaining high yields of furoic acid andadvantageously in a reduced reaction time.

Another object of the present invention is to provide a method using aheterogeneous catalyst that may be easily recycled without requiringprior treatment.

Thus, the present invention relates to a method for preparing furoicacid or a derivative thereof of formula (I):

in which R₁, R₂, R₃ and R₄ represent, independently of each other, ahydrogen atom, a linear or branched C₁-C₆ alkyl group, a —C(═O)—H groupor a —COOH group,

provided that at least one of R₁, R₂, R₃ and R₄ groups is a COOH group,

by heterogeneous catalytic oxidation of furfural or one of itsderivatives of formula (II):

in which R′₁, R′₂, R′₃ and R′₄ represent, independently of one another,a hydrogen atom, a linear or branched C₁-C₆ alkyl group or a —C(═O)—Hgroup,

provided that at least one of R′₁, R′₂, R′₃ and R′₄ groups is a —C(═O)—Hgroup,

characterized in that the oxidation is carried out in the presence of asupported catalyst based on gold nanoparticles and in a non-alkalineaqueous medium.

For the purposes of the present invention, the term “oxidation” meansthe conversion of at least one aldehyde —C(═O)—H function included informula (II) into at least one carboxylic —COOH function included informula (I).

In the context of the present invention, a non-alkaline aqueous medium(or non-basic aqueous medium) denotes a non-alkaline pH medium, i.e. apH of less than 8 and preferably not more than 6.

According to a preferred embodiment, this aqueous medium is devoid oforganic solvent.

According to a particularly preferred embodiment, this aqueous mediumconsists of water as a solvent medium.

In the context of the present invention, a C₁-C₆ alkyl group denotes analkyl group comprising from 1 to 6 carbon atoms. Such an alkyl group maybe linear or branched and may be selected from methyl, ethyl, propyl,isopropyl, butyl, isobutyl, tert-butyl, pentyl, hexyl.

According to a particular embodiment, the method according to theinvention makes it possible to obtain a furoic acid derivative offormula (I) in which at least two of the R₁, R₂, R₃ and R₄ groupsrepresent a —COOH group.

Thus, according to this embodiment, the method according to theinvention may lead to the production of a furoic acid derivative offormula (I) in which R₁ and R₂, or R₁ and R₃, or R₁ and R₄, or R₂ and R₃each represent a —COOH group, while the other two remaining groupsrepresent, independently of one another, a hydrogen atom or a linear orbranched C₁-C₆ alkyl group, preferably a hydrogen atom.

According to a preferred embodiment, the method according to theinvention makes it possible to obtain a furoic acid derivative offormula (I) in which R₁ and R₄ each represent a —COOH group, and R₂ andR₃ represent a hydrogen atom. It is then 2,5-furan dicarboxylic acid.

According to another embodiment, the method according to the inventionmakes it possible to obtain a furoic acid derivative of formula (I) inwhich R₁ represents a —C(═O)—H group or a —COOH group; R₂ and R₃represent, independently of one another, a hydrogen atom or a linear orbranched C₁-C₆ alkyl group; and R₄ is a —COOH group.

According to another particular embodiment, the method according to theinvention makes it possible to obtain a furoic acid derivative offormula (I) in which only one of the R₁, R₂, R₃ and R₄ groups is a —COOHgroup, while the other three remaining groups are, independently of eachother, a hydrogen atom or a linear or branched C₁-C₆ alkyl group.

According to another preferred embodiment, the method according to theinvention makes it possible to obtain a furoic acid derivative offormula (I) in which R₄ is a —COOH group, and R₁, R₂, and R₃ representan atom of hydrogen. It is then furoic acid itself.

Admittedly, oxidation methods of furfural using catalysts based on goldnanoparticles supported on zirconium dioxide (ZrO₂) or cerium dioxide(CeO₂) in the presence of methanol are well known, but these reactions,although carried out in the absence of a base (non-alkaline medium),lead to methyl furoate and not directly to furoic acid (Signoretto etal., Catalysts, 2013, 3, 656-670, Manzoli et al., Journal of Catalysis,2015, Vol 330, 465-473 and Pinna et al., Catalysis Today, 2013, Volume203, 196-201).

As is apparent from the experimental part which follows, the methodaccording to the invention is particularly advantageous in several ways.

More specifically, it provides access to a very high yield of furoicacid or one of its derivatives, namely a yield of the order of 98%.

The conversion of a furfural derivative of formula (II) into itsoxidation derivative of formula (I), in this case furoic acid forfurfural, is carried out in a single step in the presence of gaseousoxygen in presence of a supported catalyst based on gold.

The method according to the invention makes use of catalyticformulations that are very effective and reactive in a non-basic medium.

This advantage allows direct access to the acid form of the compound offormula (II) and not to its alkaline salt. The subsequent step ofconverting this salt into acid is no longer required and the productionof a large amount of salts that are of little or no value may beavoided.

In addition, the inventors have discovered that no leaching phenomenonoccurs during catalytic tests with this type of catalyst

In fact, the gold contents in the solutions after 4 hours and 15.5 hoursof reaction were measured by ICP-OES (measurements by induced plasmaanalysis—optical emission spectrometry). They were found to be below thedetection limit of the analyzer indicating that the passage of gold fromthe solid catalyst to the solution (a phenomenon known as leaching) doesnot occur for catalysts that are suitable for the present invention.

According to an alternative embodiment, the method of the inventioncomprises at least the steps of:

-   -   (a) having a non-alkaline aqueous solution containing at least        one furfural derivative of formula (II);    -   (b) contacting the derivative of formula (II) of the medium (a)        with gaseous oxygen in the presence of at least a        catalytically-effective amount of supported gold nanoparticles        and under non-alkaline conditions that are conducive to the        oxidation of the furfural derivative of formula (II) to the        furoic acid derivative of formula (I).

Preferably, step (b) is carried out with stirring and under a pressureof the order of 15 bars (15·10⁵ Pa) by heating the assembly to atemperature between 70° C. and 150° C., preferably between 90° C. to120° C., more preferably 110° C.

The time (or duration) of reaction is adjusted to obtain a yield offuroic acid derivative of formula (I) equal to at least 80%.

This reaction time may advantageously be only 1 to 4 hours.

According to a particular embodiment, the catalyst used is goldsupported on zirconium dioxide.

According to another particular embodiment, the catalyst used is goldsupported on hydrotalcite.

Advantageously, as illustrated in the experimental part below, themethod according to the invention makes it possible to obtain a furoicacid derivative of formula (I) and more particularly furoic acid with ahigh conversion, yield, selectivity and carbon balance.

For the purpose of the present invention, the terms “conversion rate” or“conversion” denote the ratio of the number of moles of furfuralderivative of formula (II) reacted, divided by the number of moles ofthe furfural derivative of formula (II) initially introduced.

By “selectivity” is meant the ratio of the number of moles of furoicacid derivative of formula (I) obtained at the end of the reactiondivided by the number of moles of the furfural derivative of formula(II) reacted.

By “yield” is meant the ratio of the number of moles of furoic acidderivative of formula (I) obtained at the end of the reaction divided bythe number of moles of the furfural derivative of formula (II) initiallyintroduced.

By “carbon balance” is meant the ratio of the number of carbon atomspresent in the reactor at the end of the reaction divided by the numberof carbon atoms initially present in the reactor.

Another advantage of the present invention is that the operatingconditions are simple to implement and allow the direct production ofthe furoic acid derivative of formula (I) free of any salt, because ofthe absence of base in the reaction medium.

The absence of base in the reaction medium also makes it possible toavoid the degradation of the catalyst support and the leaching of metalparticles in the liquid phase, which renders the catalyst recyclable forseveral successive uses in a closed or stable reactor under flow in openreactor.

The present invention also provides a composition comprising at leastfurfural and supported gold nanoparticles.

According to an alternative embodiment, this composition also contains afurfural derivative of formula (II) and, if appropriate, water.

Advantageously, the composition is non-alkaline.

Method According to the Invention

Definition of Gold Nanoparticles

The gold nanoparticles that are suitable for the invention are wellknown and are already commonly used in chemistry as catalysts forreactions of the hydrogenation or oxidation type as well as, inparticular, in optics, electronics, pharmacology, diagnostics ortherapy.

For most of these applications, the nanoparticles are attached to asolid support.

Various methods are known for the preparation of gold nanoparticles, inparticular in a confined mineral medium or in a confined organic medium.

The preparation of gold nanoparticles in a confined inorganic medium maybe carried out in inorganic suspensions (titanium, silica, clay), byreduction of a gold precursor in the presence of a catalyst such as thatdescribed by K. Nakamura et al. (J. Chem. Eng. Jap., 2001, 34,1538-1544). The preparation of gold nanoparticles in a silica matrixbearing hydroxyl groups by spontaneous reduction of a gold precursor isdescribed by P. Mukherjee et al. (Chem. Mater., 2002, 14, 1678-1684),and by T. Yokohama et al. (Journal of Colloid and Interface Science,2001, 233, 112-116).

The gold nanoparticles that are suitable for the invention have a sizeof between 3 nm and 15 nm, preferably between 5 nm and 10 nm.

As stated above, the gold particles are supported.

By way of illustration of the supports which are suitable for theinvention, mention may be made, in particular, of zirconium dioxide(ZrO₂) and hydrotalcite.

Thus, according to one embodiment, a catalyst that is suitable for theinvention is gold supported on zirconium dioxide or on hydrotalcite.

According to a particular embodiment, the catalyst used is goldsupported on zirconium dioxide.

The percentage by weight of gold in the Au/ZrO₂ catalyst for theoxidation of furfural is between 1% and 7% by weight, and is preferablyequal to 3% by weight, wherein zirconium dioxide is used as the supportwith a low specific surface of less than or equal to 10 m²/g.

As demonstrated in the experimental part below, this percentage of 3% byweight makes it possible to obtain optimum selectivity.

A relatively low percentage by weight of gold makes it possible todisperse the gold on the ZrO₂ surface and to increase the amount ofactive sites. For higher percentages, the formation of gold aggregateson the surface is possible, which may result in the formation of a lessactive catalyst.

When the catalyst used is Au/ZrO₂, the molar ratio of furfural/Au forthe oxidation of furfural is between 6 and 34, and preferably this molarratio is 6.

According to another particular embodiment, the catalyst used is goldsupported on hydrotalcite.

The percentage by weight of gold in the Au/hydrotalcite catalyst for theoxidation of furfural is between 1% and 3% by weight, and is preferablyequal to 2% by weight, wherein hydrotalcite is used as the support, witha low specific surface of less than or equal to 10 m²/g.

As explained in the examples below, this percentage of 2% by weightmakes it possible to obtain not only a high yield but also highselectivity in furoic acid.

When the catalyst used is Au/hydrotalcite, the furfural/Au molar ratiofor the oxidation of furfural is between 22 and 50, preferably thismolar ratio is 22.

The supported catalyst according to the invention may be prepared by anyconventional method.

For example, a supported catalyst according to the invention may beprepared by dissolving a required amount (24.8 mg) of chloroauric acidhydrate (also called tetrachloroauric acid hydrate), in particular thatsold under the name HAuO₄ by the company Alpha Aesar in a suitableamount of water (20 ml). Other gold salts may also be used for this typeof preparation, such as nitrates, especially Au(NO₃)₃ or chlorides suchas in particular AuCl₃ or Au₂Cl₆.

Then a suitable amount (1 g) of support such as zirconium dioxide orhydrotalcite is added to this solution and the mixture is stirred at atemperature between 25° C. and 50° C., preferably at room temperature(25° C.) for a period of at least 10 minutes.

Then, an amount (2 ml) of hydrazine (N₂H₄), for example that marketed in78-82% aqueous solution by Sigma Aldrich (hydrazine hydrate) is injectedinto the solution. The reduction of gold is then effected spontaneously.

The solution is subsequently subjected to stirring for a period ofbetween 30 and 60 minutes, preferably for 40 minutes at a temperature ofbetween 25° C. and 50° C., preferably at room temperature (25° C.) untilit obtains a precipitate which is filtered and then washedappropriately.

The catalyst is obtained in solid form and then dried in an oven at atemperature between 60° C. and 100° C., preferably at 80° C., for aperiod of between 6 and 14 hours, preferably overnight.

An example of preparation of a catalyst that is suitable for theinvention is more particularly described in Example 1 below.

Reaction Conditions

The oxidation may be carried out in any manner known to those skilled inthe art, in particular under oxygen or air pressure, preferably underoxygen pressure.

Preferably, the oxygen or air pressure used is such that the molar ratio02/furfural derivative of formula (II) is greater than 2.

The pressure may especially be 10 to 20 bar (10·10⁵ to 20·10⁵ Pa) ofair, preferably 15 bar (15·10⁵ Pa) of air.

The oxygen partial pressure may be from 5 to 20 bars (i.e. 5·10⁵ to20·10⁵ Pa), preferably from 10 to 15 bars (10·10⁵ to 15·10⁵ Pa), moreparticularly 15 bars (15·10⁵ Pa).

Typically, the reaction temperature for the oxidation of the furfuralderivative of formula (II), in particular furfural, is between 70° C.and 150° C., in particular between 90° C. and 120° C., and preferably110° C.

However, a temperature above 120° C. is not indicated insofar as it islikely to cause the degradation of the furfural and/or the product, theformation of various carbon compounds, and a decrease in the carbonbalance of the reaction. However, the deposition of carbon on the metalcatalysts may cause deactivation of the catalyst.

The duration of the reaction for the oxidation of the furfuralderivative of formula (II), in particular furfural, is between 1 hourand 15.5 hours, preferably between 2 hours and 4 hours. As detailed inthe experimental section below, it was observed that a reaction timelonger than 4 hours does not significantly increase the yield of thisreaction and after a period of 15.5 hours, the furfural becomes unstableand the quantity of secondary products is no longer negligible.

The method may be implemented in continuous mode or in batch mode.

In an advantageous embodiment mode, the method according to theinvention is implemented in batch mode.

In general, the method according to the invention may be appliedindustrially to the oxidation of furfural and its derivatives to obtainthe corresponding carboxylic acid free of any salt.

The expressions “comprised between . . . and . . . ” and “from . . . to. . . ” are to be understood as inclusive terms, unless otherwisespecified.

The examples which follow will make it possible to better understand theinvention, without, however, being limiting in nature.

EXAMPLES Example 1: Preparation of Catalysts Suitable for the Invention

a) Catalyst 3% Au/ZrO₂

66.5 mg of chloroauric acid hydrate (Au 49% min., from the company AlphaAesar, 99.9%) are dissolved in 20 ml of water. 997 mg of zirconiumdioxide, monoclinic with a baddeleyite structure from Sigma Aldrich, areadded to this solution. The mixture is then stirred at room temperature(25° C.) for 10 minutes.

2 ml of hydrazine (N₂H₄, 80% aqueous solution, from Sigma Aldrich) arethen injected into the solution. The reduction of gold then takes placespontaneously.

The solution is stirred for 40 minutes at room temperature (25° C.)until a purple precipitate is obtained. The precipitate thus obtained isthen filtered under vacuum and washed 3 times with water (3 times 20 ml)and once with acetone (20 ml).

The solid thus obtained is subsequently dried overnight in an oven at80° C.

b) Catalyst 2% Au/hydrotalcite

44.4 mg of chloroauric acid hydrate (Au 49% min., from the company AlphaAesar, 99.9%) are dissolved in 20 ml of water. 912 mg of hydrotalcitesynthesized in the laboratory with a low specific surface area of 10m²/g are added to this solution. The mixture is then stirred at roomtemperature (25° C.) for 10 minutes.

2 ml of hydrazine (N₂H₄, 80% aqueous solution, from Sigma Aldrich) arethen added to the solution. The reduction of gold then takes placespontaneously.

The solution is stirred for 40 minutes at room temperature (25° C.)until a purple precipitate is obtained. The precipitate thus obtained isthen filtered under vacuum and washed 3 times with water (3 times 20 ml)and once with acetone (20 ml).

The solid thus obtained is subsequently dried overnight in an oven at80° C.

Example 2: General Procedure for the Oxidation of a Furfural Derivativeof Formula (II) and More Particularly of Furfural

The procedure detailed below is the general procedure used for thecatalytic tests whose results are set forth in Example 3 of the presentinvention.

The catalytic tests are carried out in a 50 ml autoclave reactorequipped with a thermocouple (Top Industrie Autoclave 2456). Theprocedure for a standard test is detailed below.

The amount is adjusted according to the test to be performed, forexample 50 mg (from 10 to 150 mg) of furfural are added to distilledwater (10 ml) and stirred magnetically for 10 minutes. 9 ml of thefurfural solution are then added to the autoclave reactor underatmospheric pressure and at room temperature (25° C.).

100 mg of catalyst (variable nature according to the test carried out)are then added to the reaction mixture at ambient temperature (25° C.).The catalysts suitable for the invention to be tested, namely Au/ZrO₂and Au/hydrotalcite are those prepared according to the protocol ofExample 1.

10 ml of distilled water are again added to the reactor at ambienttemperature (25° C.). The reactor is then closed and the magneticstirring is set at 900 rpm at room temperature (25° C.). Then, thereactor is purged three times with oxygen. The 02 pressure is then setat 15·10⁵ Pa (15 bar) and the reactor is closed. The temperature is thenset to the desired value according to the test to be performed, namely90° C., 110° C. and 120° C., as indicated in the examples below.

The reaction time imposed for these tests may also be variable dependingon the parameter tested, as specified below; it may be 1 hour, 2 hours,4 hours or 15.5 hours.

Then, the reactor is cooled to room temperature (25° C.) and theresulting solution is taken from the reactor, centrifuged to separate itfrom residual solid catalyst particles and analyzed by HPLC (highperformance liquid chromatography) whose protocol is detailed below.

HPLC Analysis

The reaction mixture is filtered using an HPLC filter (2.5 μm), and thendiluted 5 times with water. The analysis by liquid chromatography iscarried out on a Shimadzu UFLC-MS 20-20 HPLC chain equipped with aPhenomenex Synergi 2.5 μm Hydro-RP 100 A column. The column is purged at0.5 ml/min at room temperature. (25° C.) with a 0.1% trifluoroaceticacid aqueous solution as the mobile phase, for 12 minutes. The retentiontimes for each compound are verified using commercial standards. Theconversion, yields and selectivity are determined by the calibrationcurve method (performed for each compound).

The conversions are calculated as follows:

$\frac{{Furfural}_{Tinitial} - {Furfural}_{Tfinal}}{{Furfural}_{Tinitial}} \times 100$

While the yield is calculated as follows:

$\frac{{Product}_{Tfinal}}{{Furfural}_{Tinitial}} \times 100$

The reaction rate is calculated as follows:

$\frac{{Furfural}_{Tinitial} - {{Furfural}_{Tfinal}({mmol})}}{{time}\mspace{11mu} {\left( \min \right) \cdot {weightAu}}\mspace{14mu} ({mg})}$

Example 3: Catalytic Performance Observed with a Supported CatalystBased on Gold

3.1 Influence of Temperature

The oxidation method as described above is carried out with the Au/ZrO₂catalyst according to the invention as synthesized as an example 1a).The percentage by weight of gold in this catalyst is 3% by weight.

The O₂ pressure, the stirring speed and the amount of catalyst imposedare those indicated in Example 2. The amount of furfural used is 50 mg,and the reaction time is 1 hour.

The reaction temperatures tested are 90° C., 110° C. and 120° C.

For each of these reaction temperatures, the conversion rate offurfural, furoic acid yield, furoic acid selectivity and carbon balanceare summarized in Table 1 below.

TABLE 1 Furfural Yield in Selectivity in Carbon Temperature conversionrate furoic acid furoic acid balance ° C. (%) (%) (%) (%) 90 10.7 6.157.5 95.5 110 35.0 29.7 84.7 95.0 120 46.0 36.5 79.2 90.4

It appears that the best selectivity is observed with a reactiontemperature of 110° C.

3.2 Influence of Au Concentration

A preliminary test, excluding the invention, is carried out in thepresence of ZrO₂ alone, i.e. without gold nanoparticles. It should benoted that the oxidation reaction of furfural does not occur.

Then, the oxidation method as described above is carried out with theAu/ZrO₂ catalyst according to the invention. Four different percentagesby weight of gold were tested for this catalyst, namely 1%, 3%, % 5% and7% by weight. The catalysts are prepared according to the synthesisprotocol described in Example 1a), optionally adapted according to thedesired weight percentage.

The required O₂ pressure, stirring rate and catalyst amount are asindicated in Example 2. The amount of furfural used is 50 mg, thereaction temperature is 110° C., and the reaction time is 4 hours.

For each of the percentages by weight of gold, the molar ratio offurfural/Au, the conversion rate of furfural, the yield in furoic acid,the selectivity of furoic acid and the carbon balance are summarized inTable 2 below.

TABLE 2 Furfural/ Furfural con- Yield in Selectivity in Carbon Au molarversion rate furoic acid furoic acid balance Catalyst ratio (%) (%) (%)(%) 1% 100 7.0 3.9 55.5 96.9 Au/ZrO₂ 3% 30 35.0 29.7 84.7 95.0 Au/ZrO₂5% 20 45.1 35.4 78.5 90.3 Au/ZrO₂ 7% 14 38.8 32.1 82.7 93.3 Au/ZrO₂

It appears that when the gold load increases, the conversion to furfuralalso increases, but, at the same time, the impact on the selectivity israther limited, at least for loads greater than 3% by weight.

The best selectivity is observed with Au/ZrO₂ 3% by weight.

3.3 Influence of the Reaction Time

The oxidation method as described above is carried out with the Au/ZrO₂catalyst according to the invention as synthesized as in example 1a).The percentage by weight of gold in this catalyst is 3% by weight.

The O₂ pressure, the stirring rate and the amount of catalyst imposedare those indicated in Example 2. The amount of furfural used is 50 mg,and the reaction temperature is 110° C.

The reaction times tested are 1 hour, 2 hours, 4 hours and 15.5 hours.

For each of these times, the conversion rate of furfural, yield andselectivity (named Select) in furoic acid, secondary products such as2(5H)-furanone, maleic acid, and carbon dioxide as well as the carbonbalance are summarized in Table 3 below.

TABLE 3 Duration (h) 1 2 4 15.5 Conversion (%) Conversion (%) Conversion(%) Conversion (%) Furfural 35.0 51.7 68.0 91.9 conversion rate (%)Yield Select Yield Select Yield Select Yield Select (%) (%) (%) (%) (%)(%) (%) (%) Furoic 29.7 84.7 45.0 87.0 54.1 79.6 50.0 54.5 acid2(5H)furanone 0.2 0.4 0.9 1.7 1.4 2.1 8.5 9.2 Maleic 0.1 0.5 0.4 0.7 0.40.6 2.5 2.7 acid Carbondioxide 0.0 0.0 0.2 0.3 0.2 0.3 1.4 1.5 Carbon95.0 94.6 87.9 69.1 balance (%)

Too long reaction times have revealed the instability of furfural. After2 hours, the carbon balance and the selectivity in furoic acid decrease.After a period of 15.5 hours, these decreases are even greater and theamount of secondary products simultaneously increases.

3.4 Influence of the Nature of the Support

The oxidation method as described above is carried out with differentcatalysts based on gold nanoparticles, namely Au/CeO₂, Au/MgO,Au/hydrotalcite as synthesized as Example 1 b) and Au/ZrO₂. assynthesized as an example la). The Au/CeO₂ and Au/MgO catalystsaccording to the invention may be prepared by any method known to thoseskilled in the art and in particular according to those described inexample 1 above. The percentage by weight of gold in these catalysts is2% or 3% by weight, as specified in Table 4 below.

The required O₂ pressure, stirring rate and catalyst amount are as shownin Example 2. The reaction temperature is 110° C. and the reaction timeis 2 hours.

As for the amount of furfural used, it is 50 mg.

For each of these catalysts, the conversion rate of furfural, the yieldand selectivity in furoic acid, and the carbon balance are summarized inTable 4 below.

TABLE 4 Furfural Yield in Selectivity in conversion rate furoic acidfuroic acid Carbon Support (%) (%) (%) balance 3% Au/CeO₂ 56.6 38.4 67.884.3 according to the invention 3% Au/MgO 33.1 8.8 26.5 75.7 accordingto the invention 2% 78.5 71.8 91.4 93.3 Au/hydrotalcite according to theinvention 3% Au/ZrO₂ 51.7 45.0 87.0 94.6 according to the invention

It appears that low activity is observed in the case of Au supported onCeO₂ or MgO.

A comparison between the catalytic tests carried out with differentsupports shows that, by using a supported catalyst of the hydrotalcitetype, much better performances are obtained (higher conversion andselectivity).

3.5 Influence of the Molar Ratio Furfural/Au

3.5.1 Catalyst Au/ZrO₂ According to the Invention

The oxidation method as described above is carried out with the Au/ZrO₂catalyst according to the invention as synthesized as an Example 1a).The percentage by weight of gold in this catalyst is 3% by weight.

The O₂ pressure, the stirring speed and the amount of catalyst imposedare those indicated in Example 2. The reaction temperature is 110° C.and the reaction time is 4 hours.

As for the amount of furfural used, it is adjusted to obtain threefurfural/Au molar ratios of 34, 18 and 6 respectively.

For each of these ratios, the conversion rate of furfural, yield andselectivity in furoic acid, and the carbon balance are summarized inTable 5 below.

TABLE 5 Furfural Yield in Selectivity in Furfural/Au conversion ratefuroic acid furoic acid Carbon molar ratio (%) (%) (%) balance 34 68.054.1 79.6 87.3 18 78.4 66.1 84.2 89.5 6 87.8 76.6 87.3 90.0

From these results, it appears that the Furfural/Au molar ratio has astrong influence on performance.

In fact, the lower the molar ratio, the higher the conversion, theselectivity and, therefore, the yield in furoic acid are high.

3.5.2 Au/Hydrotalcite Catalyst According to the Invention

The oxidation method as described above is carried out with theAu/hydrotalcite catalyst according to the invention as synthesized inExample 1b). The percentage by weight of gold in this catalyst is 2% byweight.

The O₂ pressure, the stirring speed and the amount of catalyst imposedare those indicated in Example 2. The reaction temperature is 110° C.and the reaction time is 2 hours.

As for the amount of furfural used, it is adjusted to obtain afurfural/Au molar ratio of 22.

The conversion rate of furfural, yield and selectivity in furoic acid,and the carbon balance are summarized in Table 6 below.

TABLE 6 Furfural/Au molar ratio 22 Conversion (%) Furfural 98.2 Yield(%) Selectivity (%) Furoic acid 96.7 98.5 Carbon balance 98.5

Thus, the use of a 2% by weight Au/hydrotalcite catalyst makes itpossible to obtain a high yield of furoic acid with a high selectivity.

Example 4: Plasma Analyses Induced Possible Losses of Metal Residues

Induced plasma analyses (ICP) of the reaction solution for the tests(catalytic tests carried out with the 3% Au/ZrO₂ catalyst according tothe invention as synthesized in Example 1a) after 1 and 15.5 hoursconfirmed that no loss of metal residues occurred during the reaction.

ICP optical emission spectrometry measurements (ICP-OES) are performedon an Agilent 720-ES spectrometer. The samples are prepared by digestionusing aqua regia. The amount of gold in the solution is determined usingthe calibration curves obtained with the standard commercial solutions.

Table 7 below summarizes the data collected, including the meanintensity and RSD intensity (Relative Standard Deviation, coefficient ofvariation) after 1 hour (white) and 15.5 hours of reaction (catalytictest).

TABLE 7 Au 211,068 White = reference (after 1 hour of reaction) Averageintensity 14,286 Intensity % RSD 26,781 Measured after 15.5 hours ofreaction Average intensity 12,123 intensity % RSD 50.195 Intensity/whiteintensity ratio 0.85

1. A method for the preparation of furoic acid or of one of itsderivatives of formula (I):

in which R₁, R₂, R₃ and R₄ represent, independently of each other, ahydrogen atom, a linear or branched C₁-C₆ alkyl group, a —C(═O)—H groupor a —COOH group, provided that at least one of R₁, R₂, R₃ and R₄ is a—COOH group, by heterogeneous catalytic oxidation of furfural or aderivative thereof of formula (II):

in which R′ 1, R′2, R′3 and R′4 represent, independently of each other,a hydrogen atom, a linear or branched C₁-C₆ alkyl group or a —C(═O)—Hgroup, provided that at least one of the R′ 1, R′2, R′3 and R′4 groupsis a —C(═O)—H group, wherein the oxidation is carried out in thepresence of a supported catalyst based on gold nanoparticles and in anon-alkaline aqueous medium.
 2. The method according to claim 1, whereinR₁ and R₄ are each a —COOH group, and R₂ and R₃ are hydrogen.
 3. Themethod according to claim 1, wherein R₄ is —COOH group, and R₁, R₂, andR₃ are hydrogen.
 4. The method according to claim 1, wherein thenon-alkaline aqueous medium is a non-alkaline pH medium having a pH ofless than
 8. 5. The method according to claim 1, wherein the aqueousmedium is devoid of organic solvent.
 6. The method according to claim 1,wherein the aqueous medium consists of water as a solvent medium.
 7. Themethod according to claim 1, the method comprising: (a) providing anon-alkaline aqueous solution containing at least one furfuralderivative of formula (II); (b) contacting the derivative of formula(II) of the medium (a) with gaseous oxygen in the presence of at least acatalytically-effective amount of supported gold nanoparticles and undernon-alkaline conditions conducive to the oxidation of the furfuralderivative of formula (II) to the furoic acid derivative of formula (I).8. The method according to claim 1, wherein the oxidation is carried outunder a partial pressure of oxygen of between 5·10⁵ Pa and 20·10⁵ Pa. 9.The method according to claim 1, wherein the oxidation is carried out ata temperature between 70° C. and 150° C.
 10. The method according toclaim 1, wherein the catalyst is gold supported on zirconium dioxide oron hydrotalcite.
 11. The method according to claim 1, wherein thecatalyst is gold supported on hydrotalcite.
 12. The method according toclaim 1, wherein the catalyst is gold supported on zirconium dioxide.13. The method according to claim 12, wherein the percentage by weightof gold in the catalyst Au/ZrO₂ for the oxidation of furfural is between1% and 7% by weight.
 14. The method according to claim 13, whereinzirconium dioxyde, used as a support, has a specific surface of lessthan or equal to 10 m²/g.
 15. The method according to claim 12, whereinthe furfural/Au molar ratio for the oxidation of furfural is between 6and
 34. 16. The method according to claim 11, wherein the percentage byweight of gold in the Au/hydrotalcite catalyst for the oxidation offurfural is between 1% and 3% by weight.
 17. The method according toclaim 16, wherein the hydrotalcite, used as a support, has a specificsurface less than or equal to 10 m²/g.
 18. The method according to claim11, wherein the furfural/Au molar ratio for the oxidation of furfural isbetween 22 and
 50. 19. The method according to claim 1, wherein the sizeof the gold nanoparticles in the catalyst for the oxidation of furfuralis between 3 nm and 15 nm.
 20. The method according to claim 1, whereinthe method is implemented in continuous mode or in batch mode.
 21. Acomposition comprising at least furfural and supported goldnanoparticles.
 22. The composition according to claim 21, furthercomprising a furfural derivative of formula (II).
 23. The compositionaccording to claim 21, wherein the composition is non-alkaline.